1. Field of the Invention
The present invention relates to a solid-state imaging device having a microlens formed on a sensor portion.
2. Background of the Invention
In order to obtain a CCD solid-state imaging device having a smaller size and a higher resolution, a unit cell formed of a light sensor portion formed of a photodiode and a CCD register tends to be reduced in size.
For this end, an area of the photodiode serving as a photosensor portion is reduced, which lowers a photoelectric conversion characteristic which is one of the main characteristics of a CCD solid-state imaging device, i.e., light sensitivity.
A size of an optical system mainly employed for a commercial-purpose small-sized single-unit video camera-recorder has been practically shifted from 1/3 inch to 1/4 and may further be shifted to 1/6 inch or smaller size.
The number of pixels widely ranges from 250,000 pixels, 360000 pixels, and 380,000 pixels to 560,000 pixels.
In order to improve the light sensitivity of such CCD solid-state imaging device, a so-called on-chip microlens technique of providing a microlens at a position corresponding to each of photodiodes to efficiently converge incident light to a photosensor portion has been established and introduced.
FIG. 1 is a cross-sectional view of a general CCD solid-state imaging device having an on-chip microlens formed therein, cut along the horizontal direction, i.e., a direction perpendicular to a transfer direction of a vertical CCD register.
As shown in FIG. 1, a charge transfer portion 32 forming a vertical CCD register 40 and a photodiode forming a light sensor portion 33 are formed of a diffusion layer or the like in a semiconductor substrate 31 made of silicon. A vertical transfer electrode 34 is formed on the charge transfer portion 32 through an insulating film. A light shielding film 35 is formed so as to cover the vertical transfer electrode 34. The photosensor portion 33 is formed so as to correspond to an opening of the light shielding film 35.
Moreover, a transparent flattening film 38 is formed so as to cover the light sensor portions 33 and the light shielding film 35. A color filter 37 is formed on a surface flattened by deposition of the transparent flattening film 36.
A transparent flattening film 38 is formed on the color filter 37. Further, micro lenses 39 whose surfaces have a certain curvature are formed on the transparent flattening film 38. Thus, the CCD solid-state imaging device 30 is formed.
In FIG. 1, it is assumed that optical paths of incident light L are those which are in parallel to one another and perpendicular to the CCD solid-state imaging device 30.
In FIG. 1, reference symbols f and r respectively represent a focal distance of the micro lens 39 and a radius of curvature of thereof.
While it is surely desired to further downsize a unit cell of the CCD solid-state imaging device 30, it is difficult for a single-layer micro lens shown in FIG. 1 to efficiently converge incident light L on the photosensor portion 33.
Specifically, as shown in FIG. 1, the light L made incident on the micro lens 39 is converged on a position f0 considerably higher as compared with the light sensor portion 33. This reduces an amount of light made incident on the light sensor portion 33 and consequently lowers light sensitivity.
As the focal position is located at an upper position above the photosensor portion 33, a so-called smear is easily produced because rays of light L once converged are made incident on the vertical CCD register 32. This phenomenon is more remarkably produced as the unit cell is more downsized.
In order to improve the problem of the convergence of light, it is necessary to mainly reduce a distance (hereinafter referred to as a layer thickness) h from a surface of the photosensor portion 33 to the microlens 39 and to increase a radius r of curvature of the microlens 39.
However, certain thicknesses of the flattening transparent films 36, 38, the color filter 37 for matching divided lights for respective colors of the CCD solid-state imaging device with one another, and so on are necessary in order to form the uniform color filter 37 and the uniform microlens 39, which limits an attempt to further reduce this layer thickness h.
In order to increase the radius r of the curvature of the microlens 39, it is necessary to set a thickness of a transparent film before reflowing thinner when the microlens 39 is formed by thermal reflow of transparent resin.
However, if the film thickness of the transparent resin forming the microlens 39 is set thinner, then, as shown in FIG. 3, a lens obtained after reflowing is shaped so as to be flat at a lens upper portion, which prevents the lens having a satisfactorily reflowed shape having a certain curvature and may prevent satisfactory convergence of light.
If the film thickness of the transparent resin is set thinner, influence of steps of a ground makes it difficult to coat transparent resin uniformly.
Therefore, there is a limit to increase the radius r of the curvature to form the uniform microlens.